1. Field of the Invention
The present invention relates to chemical surface coating or etching of metals, and more particularly, to improved baths for electrolytic anodizing of metals, particularly light metals such as aluminum, magnesium or titanium.
2. Description of the Prior Art
The surface layer of metal articles are chemically converted to oxide or salt forms such as phosphate and or chromate to protect the metal from wear, corrosion or erosion or to act as an undercoating or base layer for organic finishes. Electroless chemical oxide conversion coatings are very thin and soft. While they are adequate in many cases as a protection against mild corrosion, they are normally not suitable if additionally they have to resist more severe corrosion as well as wear and abrasion. Phosphate and chromate chemical conversion coatings have the advantage of economy and speed and involve relatively simple equipment and do not require electrical power. Adequate corrosion resistance and useful paint adhesion characteristics are imparted to the surface which are entirely sufficient for many applications. These finishes are also used as temporary protective measures on aluminum articles which may require storage for an appreciable period before use.
In the case of aluminum, the chemical oxide conversion coating is thicker than the natural oxide film which forms when a freshly cut aluminum surface is exposed to the atmosphere. However, the conversion coating is still considerably thinner than the oxide films produced by anodizing and is not suitable for applications requiring hard, dense, thick coatings.
Of the numerous finishes for metals, and particularly aluminum, none are as versatile as the electrochemical oxidation and anodizing process. The dielectric aluminum oxide film produced by anodizing aluminum in boric acid solutions may be less than 1,000 A thick. In contrast, anodic coatings produced in refrigerated sulfuric acid solutions may be more than 0.005 inch (127 microns) thick. There are numerous types of anodizing electrolytes that have been employed to produce an oxide coating with useful properties. However, sulfuric acid anodizing is the most common in this country. Many millions of pounds of aluminum products for applications requiring attractive appearance, good corrosion resistance and superior wearing quality are finished by this method.
In recent years there has been a substantially increased usage of anodized aluminum in architecture and today the use of anodized curtain walls, panels, window frames and roofing materials for commercial, residential and industrial buildings accounts for a very significant part of the total area of aluminum which is treated. Since the anodic coatings for these purposes are frequently exposed to severe conditions and are often not easily accessible for adequate cleaning, substantially thick coatings must be applied and frequently it has been found more suitable to produce architectural coatings under hard anodizing conditions both in order to apply the films more rapidly and also because corrosion resistant coatings formed at low temperatures and consequently at high voltage are somewhat better.
Architectural anodic oxide coatings for external use are usually between 0.4 and 1.4 mil thick. A thin coating of about 0.1 mil may not only be ineffective but may even intensify pitting attack. The coatings are finished in a large variety of colors and surface textures, blue, gray, gold, black and silver being some of the colors most popular today for covering walls and building panels.
However, it has been found that the uniformity of color formation is not satisfactory, the finish showing gradation of color and streaking from batch to batch and within a batch. Furthermore, the density, abrasion resistance and efficiency of deposit are not totally acceptable. Since the anodizing process is a balance between the competitive dissolution and oxide formation processes, an improvement in the efficiency of coating formation would result in a saving of time, material and energy as well as decreasing the volume of waste bath to be discarded or treated to make it environmentally acceptable.